Democratic U.S. Senate hopeful U.S. Rep. Edward Markey, D-Mass., center, greets people while marching in the Worcester County St. Patrick's Parade, in Worcester, Mass., Sunday, March 10, 2013. Markey is running in a special election for the U.S. senate seat vacated by U.S. Secretary of State John Kerry, the former senior senator from Massachusetts. (AP Photo/Steven Senne) The Associated Press

The international grassroots advocacy group 350.org, which focuses on fighting climate change, has endorsed a candidate for the first time - Massachusetts Democratic Senate candidate Edward Markey.

The political wing of 350.org, 350 Action, said the group is supporting Markey, a U.S. representative and ranking member of the House Natural Resources Committee, because of his opposition to the Keystone XL pipeline.

“Rep. Markey is the only candidate in this race who is taking a principled stand on the dangerous Keystone XL tar sands pipeline,” said 350 Action Executive Director May Boeve in a statement. “In Congress, each candidate had his shot at opposing this boondoggle of a project, which, if approved, would mean more pollution, more oil spills, and more huge profits for big oil. Only one candidate did what was right and said no.”

Markey is facing U.S. Rep. Stephen Lynch in the Democratic primary for U.S. Senate.

The Keystone pipeline is a controversial plan to build an oil pipeline from Canada to the Gulf coast.

Markey, along with other Democrats, has opposed the keystone pipeline, citing its potential for pollution and the lack of a guarantee that oil transported through the pipe would remain in the United States. “The Keystone pipeline would carry some of the world’s dirtiest oil right through the middle of our country,” Markey said in a February 2012 statement. “Whether it is the carbon pollution that spews into the skies or the oil spills that could foul our drinking water, there is no dispute that the environmental consequences attached to Transcanada’s pipeline will be grave.” Markey said a bill supporting the pipeline “turns the United States into a middleman in a multinational oil deal between Canada, South America, Europe or China.”

Republicans have generally supported the pipeline, saying it will create jobs and give the U.S. a new source of energy. Many labor leaders, particularly in the construction trades, also support the Keystone pipeline because it will create jobs.

In 2011, a Republican-sponsored bill, including language supportive of the pipeline, directed Obama to make a decision on a presidential permit required for the pipeline and ensure that actions relating to the pipeline are taken on an expedited schedule. The bill passed in the U.S. House, 279 to 147, with almost every Republican voting for it and most Democrats opposing it. Markey opposed the bill. Lynch was one of 47 Democrats who supported it.

Lynch is a former ironworker and union president and is strongly pro-union. Lynch told NECN in January 2012 that he supports the pipeline. Lynch spokesman Conor Yunits said, “Congressman Lynch, like President Obama, supports an all-of-the-above energy policy that will develop renewable energy sources, reduce our dependence on foreign oil, and spur our economy.”

Obama still has not made a final decision on the Keystone project. The New York Times reported that the decision is a politically risky one, pitting environmental groups against officials in Canada, a strong U.S. ally.

Ben Wessel, the former New Hampshire Youth Vote Director for Democratic President Barack Obama, will coordinate 350 Action’s work, which will focus on youth turnout and activities demonstrating the differences between the candidates on the Keystone pipeline.